This invention relates to aluminium alloy lithographic plates.
Aluminium alloy lithographic plates may be used in the ungrained state for certain non-critical applications but are normally grained either electrochemically or mechanically to produce a roughened surface. This improves the water retentive properties of the plate surface and assists adhesion of a light sensitive coating. Electrochemical graining is usually carried out by applying an alternating potential to a sheet of aluminium alloy immersed in a dilute acid solution. It is often preceded by a light treatment in an alkaline solution, commonly a sodium hydroxide solution, to clean the sheet of residual rolling lubricant and detritus.
Electrochemical graining produces particularly good results but is expensive. When `double sided` plates are required this involves extra cost.
Many attempts have been made to produce grained lithographic plates from aluminium and aluminium alloys by a simple chemical etching process in an alkaline solution. Such a process is commercially advantageous since it can be carried out by feeding strip material continuously through a suitable alkaline solution and automatically produces double sided plates. However using commercial purity aluminium or many previously proposed alloys the results have been poor compared with electrochemical graining and not capable of producing lithographic plates of high enough quality.
Applicants have considerable experience in alkaline etching of aluminium and many of its alloys for other purposes, such as architectural products, where coarser graining is acceptable. When commercial purity aluminium is etched in an alkaline solution the disolution process is under cathodic control (i.e.) the rate of reaction is largely controlled by the cathodic process which is exemplified by the rate of hydrogen evolution. It is known that the reaction rate can be improved markedly by the alloying additions of elements more electropositive than aluminium. The precise mechanism of operation is not fully understood but we believe that the intermetallic particles so produced facilitate the hydrogen evolution process and encourage pitting in the aluminium surrounding each intermetallic particle. Accordingly experiments have been conducted on aluminium alloys including iron, tin and manganese to accelerate the etching process. These elements produce an acceleration of the etching process but are difficult to introduce in a dense and uniform distribution and do not result in a sufficiently dense distribution of pits.
A further approach which may be used in conjunction with that described above is to make additions of elements more electronegative than aluminium and it is known that additions of magnesium result, after alkaline etching, in more uniformly and densely pitted surfaces. In particular additions of magnesium and silicon are known for architectural products and when etched provide a matt surface although this surface is too coarse for lithographic plates. Again the precise mechanism is not fully understood but we believe that the etching solution preferentially attacks the magnesium silicide intermetallic particles and then further preferentially attacks the aluminium within the pits so formed.
Alloys of aluminium and calcium have been known for many years but until recently they were little used and then mostly for cast products where good heat/strength characteristics were desirable. More recently aluminium/calcium alloys with the calcium addition at or near the eutechtic have been used for their predictable superplastic properties. None of this previous use points to any benefit of adding calcium to aluminium to enhance alkaline etching characteristics. However we have found that certain alloys of aluminium and calcium are capable of being etched in alkaline solution to be usable as lithographic plates.